Deserts & Aeolian Processes. Distribution and causes of dry lands Dry regions cover 30 percent...

Deserts & AeolianProcesses

Distribution and causes of dry lands

• Dry regions cover 30 percent of Earth’s land surface

• Two climatic types are commonly recognized

• Desert or arid• Steppe or semiarid

WIND AND WAVES

AOLIAN: WIND LANDFORMS

COASTAL LANDFORMS

WIND EROSIAN

• SALTATION: SAND GRAINS MOVED BY THE WIND IN A SERIES OF JUMPS AND

AND BOUNCES.

WIND EROSIAN

• DEFLATION: THE PROCESS BY WHICH WIND REMOVES THE TOP LAYER OF FINE VERY DRY SOIL OR ROCK PARTICLES AND LEAVE BEHIND LARGE ROCK PARTICLES. THESE PARTICLES OFTEN FORM A DENSLY PACKED SURFACE CALLED DESERT PAVEMENT

Wind Erosion

• Two forms of wind erosion– Deflation

• Removal of material by wind transport

– Abrasion

• Mechanical weathering of rock by wind blown sand

DEFLATION PROCESS

blowout and pillar formationdeserts: wind action

wind picksup dry finesediment

and createsblowout

vegetation holdssedimentin place

and formspillar

DEFLATION AND DESERT PAVEMENT

Wind Erosion

• Two forms of wind erosion– Deflation

• Removal of material by wind transport

– Abrasion

• Mechanical weathering of rock by wind blown sand

WIND EROSIAN

• VENTIFACTS: ROCKS PITTED, GROOVED OR POLISHED BY WIND ABRASION

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…wind blowing in one direction abrades up wind rock faces…

wind

ventifacts

deserts: wind actiondust suspended by wind…larger sand grains move by saltation (bouncing)…

formed by sand grains moving in high speed winds

ventifacts

“sand blasting”

Wind in the desert

• Wind deposits• Two types of wind deposits

– Dunes» Slow migration of dunes in the direction

of wind movement» Several types of sand dunes including

barchan, transverse, longitudinal, parabolic and star dunes

dunes

deserts: wind actionsand dunes: mounds of loose sand piled up by wind

• large sand supply• winds that blow in same direction

sand erodes from andblows up the

slope facing the wind

sand falls down thedown wind slope

(slipface)and deposits

(calm air)

dune migrates through time

WIND DEPOSITION

• DUNES: MOUNDS OF WIND DEPOSITED SAND

• TYPES: BARCHAN: CRESCENT SHAPED DUNES

• PARABOLIC DUNES: U SHAPED DUNES FORM WHEN PLANTS ANCHOR THE EDGES OF BARCHAN DUNES CAUSING AN INVERTED U SHAPE

types of sand dunes

• crescent shapedbarchan

• horns point down wind• steep slip face on concave side

one direction of wind and limited sand supply

types of sand dunes

• deeply curvedparabolic

• horns anchored by vegetation• convex in down wind direction (opposite to barchans)

one direction of wind and large sand supply

types of sand dunescontrolled by dominant wind direction(s) and amount of sand

barchan parabolic

transverse longitudinal

ancient sand dunes

Sand dune migration

WIND DEPOSITION

• LONGITUDINAL DUNES: LIE PARALLELL TO THE WIND

• TRANSVERSE DUNES: LIE PERPENDICULAR TO THE WIND (THESE LOOK LIKE WIND RIPPLES)

types of sand dunes

• extremely long, high (100’s of meters), and straightlongitudinal

• regularly spaced• area between dunes is swept clean of sand

large sand supply -- mechanism not fully understood

• cross winds may be important

• long axis parallel to prevailing wind direction

types of sand dunes

• straighttransverse

• elongate• steep slip face

one direction of wind and large sand supply

• Wind deposits• Significant depositional landforms are

created by wind in some regions• Two types of wind deposits

– Dunes» Mounds or ridges of sand» Often asymmetrically shaped» Windward slope is gently inclined and

the leeward slope is called the slip face

• Wind deposits– Loess

» Blankets of windblown silt» Two primary sources are deserts and

glacial outwash deposits» Extensive deposits occur in China and

the central United States

sand pillar

deserts: wind actionloess: wind deposited clay and silt sized particles

…sources are glacial outwash and playas

forms fertile soils that erode easily

Dust Bowl of 1930’s produced byerosion of loess during drought

global distribution of loess deposits

deserts: wind actionimportant agent for transportation and erosion

…caused by large daily temperature and pressure changes…

dust stormsoccur if fine grained sediments are available

dust storm